About Optics & Photonics TopicsOSA Publishing developed the Optics and Photonics Topics to help organize its diverse content more accurately by topic area. This topic browser contains over 2400 terms and is organized in a three-level hierarchy. Read more.

Topics can be refined further in the search results. The Topic facet will reveal the high-level topics associated with the articles returned in the search results.

Abstract

The secure distribution of a secret key is the weakest point of shared-key encryption protocols. While quantum key distribution schemes could theoretically provide unconditional security, their practical implementation remains technologically challenging. Here we provide an extended analysis and present an experimental support of a concept for a classical key generation system, based on establishing laser oscillation between two parties, which is realized using standard fiber-optic components. In our Ultra-long Fiber Laser (UFL) system, each user places a randomly chosen, spectrally selective mirror at his/her end of a fiber laser, with the two-mirror choice representing a key bit. We demonstrate the ability of each user to extract the mirror choice of the other using a simple analysis of the UFL signal, while an adversary can only reconstruct a small fraction of the key. The simplicity of this system renders it a promising alternative for practical key distribution in the optical domain.

Figures (6)

(a). Schematic of the UFL system. (b). Top: simulated steady state UFL spectra for the four possible combinations of mirror choices by Alice and Bob. The spectra for (0,1) and (1,0) mirror choices are distinguishable only in their weak spectral side lobes. Bottom: reflectivity profiles |r0(ω)|2, |r1(ω)|2 of ‘0’ and ‘1’ mirrors used in simulations: r0(ω)=0.75°sinc2(ω/Δω), r1(ω)=0.75°sinc2[(ω-ωsep)Δω], with a spectral width of Δω=2π°7GHz, and a frequency separation of ωsep≡2π(f1-f0)=2π°5 GHz. The EDFAs small signal gain, saturation power and noise figure are: 10log10G0=17dB above transparency, Psat=13dBm and NF=3dB.

(a). Simulated time resolved spectra of the UFL signal, with mirrors choice of (0,1) corresponding to a ‘0’ bit. The spectra were calculated after 3 (red), 6 (magenta) and 10 (blue) one-way propagation cycles following the UFL switch-on. (b). Simulated histograms of the difference between the power in the left hand side lobe and that of the right hand side-lobe, 3 ms following switch-on of a 25 km long UFL. Using such time-resolved spectral measurements, Eve can recover 95% of the key.

Simulated histograms of the difference between the power in the left hand side lobe and that of the right hand side-lobe, 3 ms following the switch-on of a 25 km long UFL. The terminals include intermediate narrowband filters, with a 3 dB full width of 2.5 GHz and a 20 dB full width of 3.75 GHz. The small signal gain of the EDFAs was reduced to 10log10G0=7 dB. In addition, the peak reflectivity frequencies of the mirrors were randomly varied between bits, within a range of 2.5 GHz surrounding the nominal values. (a). One filter included in each terminal. (b). Two filters cascaded in each terminal

(a). Alice and Bob’s decision variable VAB(t), versus time following switch-on. Significant signal power is observed when Alice and Bob share a secure key bit (blue, complementary mirror choices), no signal is observed when information represented by mirror choices is non-secure (green, identical mirror choices). (b). Histogram of the RMS value of VAB(t), taken 3 ms after the switch-on of the UFL. Blue: the decision variable distribution for secure bits, (1,0) and (0,1) mirror choices. Red: the distribution for non-secure bits, (1,1) and (0,0) choices. Setting a threshold value for VAB(t), 994 out of 1000 bits are properly categorized.